1. Field of the Invention
This invention relates to a carrier for a combustion catalyst, to a catalyst for combustion, and to processes for producing and using the same. Particularly it relates to a catalyst carrier which maintains a high activity and retains a high specific surface area at elevated temperatures, and to a process for producing the same.
2. Description of the Prior Art
In recent years, research and development on making combustors highly efficient and compact have been actively advanced in various fields by applying the so-called catalytic combustion principle, wherein combustion is promoted using a catalyst, to various combustors such as gas turbines. Usually it is necessary for the catalyst used in such combustors to maintain its activity at high temperatures of 1,000.degree. C. or higher.
Furthermore, in various petrochemistry-related industries, too, there is a tendency to use such catalysts at higher temperatures for yield improvement or for producing new products; hence improvement in heat resistance of such catalysts has been a serious problem for their development.
The factors governing the heat stability or resistance of catalysts varies depending on the catalyst, but in the case of a catalyst having an active ingredient supported on a carrier, the heat stabilities of the carrier and the active ingredient govern the heat resistance. Particularly in the case of a catalyst used at temperatures exceeding 1,000.degree. C., it is indispensable for enhancing the heat resistance that the carrier barely sinter and retain a high specific surface area at high temperatures. Thus, various inventions directed to oxides which are stable at high temperatures, that is, carriers having a high melting point and a specific surface area have been made. For example, stabilized zirconia (ZrO.sub.2), composite oxides such as mullite (2Al.sub.2 O.sub.3 .multidot.3SiO.sub.2), spinel (MgAl.sub.2 O.sub.4), La-containing .beta.-Al.sub.2 O.sub.3, etc., have been studied (Japanese Patent Application No. Sho 59-92866/1984).
However, it is the present state of the art that these carriers are not regarded as having those characteristics necessary to obtain a high performance catalyst. For example, zirconia, mullite, etc., have a tendency that when they are kept at high temperatures of 1,000.degree. C. or higher for a long time, their specific surface areas are gradually reduced. La-containing .beta.-Al.sub.2 O.sub.3 has superior properties of retaining a high specific surface area at high temperatures, but lanthanum compounds are expensive as raw materials, and moreover there is a problem that unless it is prepared from an aqueous solution of a lanthanum salt and an aluminum salt, according to a complicated coprecipitation method, its characteristics cannot be fully exhibited.